Various methods are known as a method for producing cycloolefins. Among others, a method for partially hydrogenating a monocyclic aromatic hydrocarbon using a ruthenium catalyst is most typical. Many results of the investigation on catalyst components, types of carriers, metal salts as additives to the reaction system, or the like are reported as a method for increasing yield (conversion×selectivity).
For a reaction system containing both water and zinc which gives a relatively high yield of cycloolefin, there are proposed several methods including (1) a method in which a catalyst prepared by loading particles mainly composed of metallic ruthenium having an average crystallite size of 3 to 20 nm on a carrier is used as a catalyst for partially reducing monocyclic aromatic hydrocarbons with hydrogen in the presence of both water and at least one zinc compound under a neutral or acidic condition (Patent Document 1); (2) a method in which all of at least one of a zinc oxide and a zinc hydroxide in an amount of the saturated solubility thereof or less is present in a dissolved state for producing cycloolefins by partially hydrogenating monocyclic aromatic hydrocarbons in the presence of a ruthenium catalyst (Patent Document 2); (3) a method in which hydrogenation catalyst particles having an average crystallite size of 20 nm or less mainly composed of metallic ruthenium are used for partially reducing monocyclic aromatic hydrocarbons by hydrogen in the presence of water, the reaction being carried out in the presence of at least one solid basic zinc salt under a neutral or acidic condition (Patent Document 3); and the like.
When a method for producing cycloolefins by partially hydrogenating monocyclic aromatic hydrocarbons with hydrogen using a ruthenium catalyst is industrially carried out, the catalyst can be preferably used for a long period of time so as to reduce the frequency of replacing the catalyst, which will inhibit efficient production if it is high. However, it is known that when the catalyst is used for a long period of time, the activity of the catalyst decreases. A decrease in activity of the catalyst caused by a physical change (e.g., sintering) of the active site of the catalyst itself due to reaction circumstances (temperature and reaction heat), caused by the accumulation of a catalytic poison (e.g., a sulfur compound or a foreign metal), caused by the assumed interaction between hydrogen and ruthenium, or the like is reported.
For example, Patent Document 4 describes an example of poisoning by a sulfur compound, and Patent Document 5 describes an example of poisoning by iron. Further, Patent Document 6 discloses an example of a method for regenerating a ruthenium catalyst poisoned by a sulfur compound.
As a method for regenerating the ruthenium catalyst whose activity has decreased due to the interaction between hydrogen and the ruthenium catalyst, there are proposed a method of bringing the ruthenium catalyst into contact with oxygen in a liquid phase (Patent Document 7); a method of maintaining the catalyst at a hydrogen partial pressure lower than that at the hydrogenation and at a temperature not lower than the temperature lower by 50° C. than the hydrogenation temperature (Patent Document 8); a method comprising the steps of bringing the ruthenium catalyst into contact with oxygen in a liquid phase and maintaining the catalyst at a hydrogen partial pressure lower than that at the hydrogenation and at a temperature not lower than a the temperature lower by 50° C. than the hydrogenation temperature (Patent Document 9); and the like. Furthermore, in Patent Document 10, a method and an apparatus are proposed in which a part of the catalyst in continuous reaction is continuously or intermittently taken out the reactor for carrying out regeneration treatment and returned again to the reactor for carrying out partial hydrogenation.
In addition, it is also known that when the catalyst is used for a long period of time, the selectivity of cycloolefins varies with time. For the purpose of avoiding the variation of the selectivity, there have been proposed a method in which the reaction is carried out while changing the concentration of a metal sulfate in the water phase (Patent Document 10), a method in which sulfuric acid is added to the reaction system (Patent Document 11), and the like.
On the other hand, Patent Document 12 describes a method for producing cycloolefins using a catalyst prepared by adjusting the chloride ion concentration in the catalyst to 400 wt ppm or less based on 1 part by weight of ruthenium and then bringing the catalyst into contact with hydrogen. This document describes that a chloride as well as a nitrate, a sulfate, an acetate, a phosphate, and the like of a metal such as zinc, manganese, and cobalt can also be used as a metal salt that can be contained in water during reaction.
Further, Patent Document 13 describes a method for producing cycloolefins using water having an electric conductivity of 1 μS/cm or less. Similar to Patent Document 11, this document describes that a chloride as well as a nitrate, a sulfate, an acetate, a phosphate, and the like of a metal such as zinc, manganese, and cobalt can also be used as a metal salt that can be contained in the water phase during reaction.
However, in all of the above-described methods, there remains a problem that a long-term use of the catalyst may cause the physical change of the active site of the ruthenium catalyst itself, leading to the decrease in the selectivity of cycloolefins. Investigations by the present inventors have revealed that the presence of chloride ions in the reaction system not only causes decrease in the catalytic performance but also leads to a problem that the regeneration treatment of the catalyst cannot restore the activity thereof. None of the above-described documents mention anything about the influence of chloride ions present in the reaction system on the catalytic performance.    Patent Document 1: JP-B-8-25919    Patent Document 2: JP-B-5-12331    Patent Document 3: JP-B-8-19012    Patent Document 4: JP-B-2-19096    Patent Document 5: JP-B-7-39353    Patent Document 6: JP-B-3-41216    Patent Document 7: JP-B-2634828    Patent Document 8: JP-B-2886563    Patent Document 9: WO 97/16249    Patent Document 10: JP-A-2001-26556    Patent Document 11: JP-A-10-330294    Patent Document 12: JP-B-3125913    Patent Document 13: JP-A-8-259473
None of the above-described prior documents mention how to solve the problem of the decrease in the activity of the ruthenium catalyst due to the physical change (e.g., sintering) of the active site of the ruthenium catalyst itself by a long-term use of the catalyst and describe the influence of the change of the active site on the change of the conversion or selectivity of cycloolefins.
It is an object of the present invention to provide a method for suppressing physical change occurring in the active site of a ruthenium catalyst when the ruthenium catalyst is used for a long period of time, in particular, a method capable of suppressing the decrease in the activity and cycloolefin selectivity after the catalyst is subjected to a known catalyst regeneration treatment.